HIV assays have not yet been standardized, and a comparison between assays, although important, is not yet dependable. For example, a series of five papers in Nature evaluating soluble CD4 (sCD4) as a blocker of HIV infectivity were divided two and three, for and against the efficacy of the blocking properties of sCD4. With these results in mind, we developed a theory to account for the hidden variables influencing the outcome of the experiments. In our model, four unknown parameters characterize the virus under assay conditions: shedding and inactivation rates, an infection rate, and the association constant for gpl2O-sCD4 interaction. These parameters can be determined by experiments following certain protocols. Peter Nara is currently carrying out some of these protocols at FCRF, and the results show that viral infectivity assays and physical methods produce similar estimates of the gpl2O-sCD4 association constant. This is a considerable saving of experimental effort since the viral infectivity assays are much less laborious than the physical methods. Further experiments characterizing various viral strains and their tropism for different cell lines are planned.